Sno-Isle Libraries is a public library system serving Island and Snohomish counties in the U.S. state of Washington. The system is among the largest in Washington state and has an annual circulation of 11 million materials. The library's 23 branches and bookmobile services reach every incorporated city in the two counties, with the exception of Everett (which retains its own municipal system) and Woodway. Sno-Isle was formed in 1962, from the merger of two systems serving each county that were established in 1944 and 1962.

While cities in Island and Snohomish counties established their own libraries in the early 20th century, the first inter-city system was created by voters in unincorporated Snohomish County in 1944. The system's creation was spurred by the state legislature's approval of rural library districts in 1941. The new Snohomish County Library was temporarily headquartered in the basement of the separate Everett Public Library before moving to another building in Everett. The system's first library was in Alderwood Manor; the first bookmobile was purchased in 1947.

The state government sponsored demonstration library and bookmobile projects on Camano and Whidbey islands in 1961, which created interest in establishing an Island County system. The Island County Rural Library District was established by voters in November 1962 and merged with the Snohomish County system on December 17, forming the Snohomish-Island Inter-County Rural Library District.

The new library system was named "Sno-Isle" to reflect the two counties. A new headquarters building near Marysville was constructed in 1965. Initially, the Sno-Isle Regional Library signed contracts with incorporated cities to operate their libraries and join the system for a fixed amount. Rural branches would rely on property taxes generated within the district, as well as donations from organizations and members of the community. The library system purchased and installed its first computerized catalog system in 1983. Incorporated cities began voting to annex themselves into the Sno-Isle district in the late 1980s, with promises of new libraries and potential cost savings over the contracted service.

All Sno-Isle branches were closed in March 2020 due to the COVID-19 pandemic, but reopened with curbside pick-up service three months later. In-person services resumed at some branches in early 2021.

In 2024, the city government of Everett proposed a consolidation of their city libraries with Sno-Isle to address a budget deficit. The Everett Public Library's board of trustees opposed the proposal, which would require voter approval.

As of 2018, the Sno-Isle Libraries system has 23 branches. They serve every city in Island and Snohomish counties, with the exception of two cities: Everett, which has its own system, and Woodway, which had contracted service until 1978. The system covers an area of 2,260 square miles (5,900 km) and a population of over 700,000 residents.

Two locations, in the Mariner area of Everett and Smokey Point area of Arlington, are "demonstration" libraries that are in leased retail spaces that precede a permanent branch. The Camano Island location was formerly a demonstration library that was replaced by a permanent branch in 2015. A permanent library for the Mariner neighborhood is planned to begin construction in 2026 as part of a mixed-use development.

Name	Annexed	Building opened	Floor space
			sq ft	sq m
Arlington		1981	5,000	460
Brier		1996	2,800	260
Camano Island		2015	4,900	460
Clinton		2000	1,300	120
Coupeville		2010	6,000	560
Darrington	2008	2009	5,000	460
Edmonds	2001	1982	20,000	1,900
Freeland		2006	4,800	450
Granite Falls		2001	6,500	600
Lake Stevens	2008	1985	2,500	230
Lakewood/Smokey Point		2018	4,000	370
Langley	2012	1923	3,500	330
Lynnwood		1999	25,900	2,410
Mariner (Everett)		2017	3,700	340
Marysville	1968	1995	23,000	2,100
Mill Creek		1992	7,400	690
Monroe	1966	2002	20,000	1,900
Mountlake Terrace	1985	1988	12,800	1,190
Mukilteo	1996	1998	15,000	1,400
Oak Harbor		1993	11,200	1,040
Snohomish		2003	23,000	2,100
Stanwood		1986	5,400	500
Sultan	2008	1999	4,400	410

The Sno-Isle Libraries system is headquartered at an administration and processing center on the Tulalip Indian Reservation, west of Marysville. It is governed by a seven-member board of trustees, of whom two are appointed by Island County and five by Snohomish County. The system is overseen by an executive director that is appointed by the board of trustees. Jonalyn Woolf-Ivory, a longtime library employee, was appointed as executive director in 2002 and retired in 2018.

The library system has annual expenditures of $77 million and revenues of $74 million. As of 2017, 98 percent of revenue was generated by a property tax levied on all properties within the district. The remaining two percent of revenue were sourced from a timber excise tax, a leasehold excise tax, contract fees from municipal governments, and donations.

In 2022, Sno-Isle had a total circulation of 7.4 million items, placing it third in Washington behind the King County Library System and Seattle Public Library. It had the state's fifth-highest turnover rate, at 4.74 checkouts per item.

• Official website
• Sno-Isle Libraries at WorldCat
• Sno-Isle Libraries at LibraryTechnology.org